Prostate Test in DoubtMost American men over 50 get an annual blood test, called a PSA, that is thought to indicate whether they may have prostate cancer. But a new study questions the accuracy of the widely used test and raises questions about how best to screen for the cancer. NPR's Richard Knox reports.

Most American men over 50 get an annual blood test called a PSA — prostate-specific antigen — that is thought to indicate whether they may have prostate cancer. On a scale of zero to 10, doctors had thought that the level four or below was safe. Now researchers report in the New England Journal of Medicine that even men with a PSA below four may have a substantial risk of cancer.

The research was part of a $73 million study funded by the National Cancer Institute. It enrolled almost 19,000 men over 55. An analysis found that men with a PSA level formally considered safe still had a 15-percent risk of prostate cancer.

Study co-author Dr. Ian Thompson, of the University of Texas in San Antonio, said the most worrisome finding was that aggressive, or "high-grade," cancers were found at all levels of PSA; the researchers were unable to find a level of PSA below which there was no risk of prostate cancer. NPR's Richard Knox reports on the implications of the study.